LOL every time I read about the weight of the stock exhaust system it seems to be getting heavier.
I weighed the cat, the three mufflers and the header all off an '06 Classic which would surely have to be one of, if not THE heaviest Rocket exhausts.
As you can see a complete stock three pipe exhaust doesn't even weigh 40lbs much less just the mufflers. So unless your new slip is made out of some kind helium - hydrogen alloy weighing less than air effectively lifting your bike off the ground then you won't be 40lbs+ lighter.
For what it's worth I have 149hp at the wheel still running three Triumph mufflers (TORS). I use an aftermarket header and cat bypass and most importantly have a fantastic tune. I could get quite a bit more power out of it by using a nice slip on certainly, but to me I love the classic look.